1. Field of the Invention
The present invention relates generally to propellers and, more particularly, to a propeller device with a vent plug which has an opening formed therethrough to allow a pre-selected flow of fluid to pass from a region within a hub of the propeller device to a region proximate the outer surface of the hub at a position near a propeller blade where the pressure is reduced when the propeller device rotates about a central axis of the propeller hub.
2. Description of the Prior Art
Many different types of propeller devices are known to those skilled in the art. In some applications, particularly if the propeller blades have a high pitch, the marine propulsion system may experience an unsatisfactory acceleration performance. For example, if the engine does not have sufficient power, a sudden throttle demand can overload the engine and, in some extreme instances, actually cause the engine to stall. There are two possible reasons for this phenomenon. First, during initial acceleration from a standstill condition, the propeller blades must accelerate an annular volume of water described by the blades as they rotate about the centerline of the propeller device. This effort is not assisted by movement of the boat through the water. Secondly, most engines exhibit a horsepower versus RPM characteristic which results in significantly decreased horsepower output from the engine at low speeds. Until the rotational speed of the engine reaches a certain magnitude, the power output from the engine is significantly reduced. Both of these circumstances exist during initial acceleration from a standstill condition.
Propeller blades with a high pitch are typically selected for high speed applications, but they can make it more difficult to accelerate the boat to a sufficient speed to achieve a planing condition. Naturally, if the engine of the boat has sufficient power, this potential acceleration problem can be overcome.
One solution to the above-described problem is to provide a ventilation hole in the hub of the propeller in order to allow exhaust gases to pass through the ventilation hole, in a radially outward direction, from within the propeller hub to a region at the outer cylindrical surface of the hub near the propeller blades. These ventilation holes are typically provided at regions where low pressure zones will be developed behind the blades as a result of the rotation of the propeller device. When ventilation holes are provided, exhaust gas is allowed to pass from the region within the propeller hub, through the ventilation, and into the water within the annular volume described by the path of the propeller blades. By displacing some of the water within this annular volume, the presence of exhaust gases creates an environment surrounding the propeller blades through which is easier for the blades to move. In other words, during acceleration of the propeller device, the presence of exhaust gases in the region swept by the propeller blades displaces water away from this region and facilitates the rotation of the propeller. This concept is well-known to those skilled in the art, and ventilation holes have been provided in several types of propeller devices.
If a propeller device with a ventilation hole is used in conjunction with an engine that has more than sufficient power to satisfy the acceleration needs of the marine vessel on which it is used, the existence of the ventilation hole can possibly cause a situation referred to by those skilled in the art as "breaking loose" by the propeller blades. The symptom of this condition is a sudden increase in speed of the engine and propeller with a corresponding decrease in the power of the engine which is being efficiently applied to move the boat through the water. Typically, the only solution to this deleterious condition is to rapidly decrease throttle demand and allow the propeller to begin to operate properly before reaccelerating.
Therefore, a propeller with a ventilation hole that will operate satisfactorily under one set of conditions will not necessarily operate properly under another set of conditions. If a propeller with a ventilation hole is used on a boat with an engine having limited power, it can be advantageous and can assist during acceleration modes. That same propeller device, however, if placed on another boat with a much more powerful engine, will likely cause "breaking loose" of the propeller blades. As a result, owners of powerful boats typically place plugs in the ventilation holes to essentially avoid the intended function of the ventilation hole. The use of plugs for these purposes creates an "all or nothing" situation where the propeller device must either be used with the full sized ventilation holes which were cast into the propeller device during initial manufacture or, alternatively, with completely blocked ventilation holes that were originally provided with the propeller device.
It would therefore be significantly beneficial if a device could be developed which allows for the originally cast ventilation holes to be modified in a manner that does not require the complete blockage of the holes when the originally cast ventilation hole is too large for a particular application.